Various constructions have been used for burial vaults in the past. A common form of burial vault is made of concrete having an asphalt inner liner or an inner liner of fiber reinforced resin. Other burial vaults have been made of molded plastic materials.
Reinforced concrete burial vaults have adequate structural and tensile strength to resist the crushing force of the overlying earth and the hydraulic pressure to which the vault is subjected when it is interred in a grave, as well as being able to withstand the impact of handling equipment. On the other hand, burial vaults made entirely of synthetic or plastic materials generally lack the strength to withstand the weight of the over-lying earth and the hydraulic pressures to which the vault is subjected.
U.S. Pat. Nos. 3,439,461 and 3,787,545 describe a concrete burial vault having an integrally bonded inner plastic liner. In accordance with the aforementioned patents, a rigid open top liner is molded from thermoplastic resin and an uncured epoxy resin adhesive is applied to the outer surface of the liner. While the epoxy resin is in an uncured state, a wet concrete mix is applied directly onto the adhesive and on curing of the adhesive and the concrete, a composite structure is formed in which the concrete is bonded to the inner thermoplastic liner.
In use of the method, as described in the aforementioned patents, positive bonding of the concrete layer to the plastic liner is generally not possible without the use of a solvent, which is capable of solubilizing the outer surface of the plastic liner to thereby permit the adhesive to thoroughly wet and combine with the thermoplastic liner. The solvents commonly used for this purpose are aromatic solvents, since they are compatible with the adhesives and are capable of solubilizing the thermoplastic liner. However, aromatic solvents tend to be hydrophobic and therefore tend to be counterproductive during the intermixing process with the wet concrete. Although good bonds can be made between the concrete and the plastic liner, as the amount of solvent and the ambient conditions must be precisely controlled in order to obtain a proper evaporation rate of the solvent and achieve the necessary concentration of solvent in the adhesive when the concrete is poured.
If solvents with relatively fast evaporation rates are used, the rapid evaporation of the solvent leaves the adhesive relatively uncured. Further, the use of rapidly evaporating solvents increases the risk of fire or explosion as these solvents are highly flammable. Because of this, the trend in industry has been to use slower evaporating solvents. However, the slower evaporating solvents will remain in the adhesive layer for a longer period of time and increase the hydrophobic characteristics of the adhesive layer. As the concrete must be poured before the adhesive cures, the use of slower evaporating solvents will result in residual solvents being present in the adhesive which reduces the bond between the concrete and the plastic liner.
Therefore, in utilizing the method as set forth in the aforementioned patents, there must be accurate and precise control of the evaporation of the solvent in conjunction with the curing of the resin and, in practice, it is virtually impossible to maintain this control due to variations in ambient temperature, humidity, local air velocity and the like.